This invention relates generally to safety latches for in swinging doors and, in particular, for a safety latch which is capable of providing the function of locking the door when it is in a closed position, locking the door when it is in a partially open position, and being totally disengaged to permit the free opening of the door.
There are many situations in which a homeowner desires to have a safety latch capable of permitting the door to be opened partially, yet still providing a secure lock in case an intruder attempts a forced entry. One of the more popular of these types of safety latches includes a safety chain having a pin with an enlarged head which engages a slot to provide the locking function. One of the disadvantages with these types of devices is that the chains provided are usually not sufficiently strong to withstand a heavy impact on the door and, therefore, do not provide the desired safety from forced entry by intruders. Numerous other devices have been described which use complex mechanisms to afford the functions desired from a safety latch. These devices include those described in U.S. Pat. No. 2,601,459 (June 24, 1952, Ross); U.S. Pat. No. 2,845,295 (July 29, 1958, Clarke); U.S. Pat. No. 2,368,411 (Jan. 30, 1945, Chambers); and U.S. Pat. No. 3,980,330 (Sept. 14, 1976, Walker). These devices have the disadvantage of either including complex mechanisms for achieving the locking function, or not providing the three functions described from a safety latch. More recently, devices have been described in U.S. Pat. No. 3,940,956 (Mar. 2, 1976, Stevens); and U.S. Pat. No. 3,924,886 (Dec. 9, 1975, Markovitch), which describe substantially complex devices incorporating multiple hinges to provide the three locking functions desired from a safety latch. The multiple hinge design, in addition to incurring additional costs to manufacture, are also rather unattractive and thus are not found in common use in most households. In another more recent patent, U.S. Pat. No. 4,042,265, a relatively simple device is disclosed capable of performing the locking function on a partially open door. The device disclosed has the disadvantage that it cannot perform the locking function on the closed door unless a thumb screw is tightened which is rather inconvenient. In U.S. Pat. No. 4,174,861 (Nov. 20, 1979, Drake), a locking device for use with a dead bolt means is disclosed, which provides three functions desired of a safety latch. The disadvantage with that device is that it requires a conventional dead bolt lock set, to engage the locking device.